Method of manufacturing wick structure for heat pipe

ABSTRACT

A method is disclosed to produce a wick structure for a heat pipe. The wick structure is a sintered powder wick and is produced by sintering process. A group of powders is firstly provided. The group of powders is then classified into many sub-groups in terms of powder size. At least one sub-group of the powders is selected to form the wick structure via the sintering process. Thus, the powders used to construct the wick structure are confined to powders having a relatively narrower range of powder size in relative to the group of powders as originally provided. This has greatly reduced the complexity involved in the sintering process, and as a result, the required sintering temperature and the required time for the sintering process are easier to be determined.

FIELD OF THE INVENTION

The present invention relates generally to an apparatus for transfer ordissipation of heat from heat-generating components such as electroniccomponents, and more particularly to a method of manufacturing a wickstructure for a heat pipe.

DESCRIPTION OF RELATED ART

Heat pipes have excellent heat transfer performance due to their lowthermal resistance, and therefore are an effective means for transfer ordissipation of heat from heat sources. Currently, heat pipes are widelyused for removing heat from heat-generating components such as centralprocessing units (CPUs) of computers. A heat pipe is usually a vacuumcasing containing therein a working fluid, which is employed to carry,under phase transitions between liquid state and vapor state, thermalenergy from one section of the heat pipe (typically referring to as the“evaporating section”) to another section thereof (typically referringto as the “condensing section”). The casing is made of high thermallyconductive material such as copper or aluminum. Preferably, a wickstructure is provided inside the heat pipe, lining an inner wall of thecasing, for drawing the working fluid back to the evaporating sectionafter it is condensed at the condensing section. Specifically, as theevaporating section of the heat pipe is maintained in thermal contactwith a heat-generating component, the working fluid contained at theevaporating section absorbs heat generated by the heat-generatingcomponent and then turns into vapor. Due to the difference of vaporpressure between the two sections of the heat pipe, the generated vapormoves towards and carries the heat simultaneously to the condensingsection where the vapor is condensed into liquid after releasing theheat into ambient environment by, for example, fins thermally contactingthe condensing section. Due to the difference of capillary pressuredeveloped by the wick structure between the two sections, the condensedliquid is then drawn back by the wick structure to the evaporatingsection where it is again available for evaporation.

The wick structure currently available for heat pipes includes finegrooves integrally formed at the inner wall of the casing, screen meshor bundles of fiber inserted into the casing and held against the innerwall thereof, or sintered powders combined to the inner wall bysintering process. Among these wicks, the sintered powder wick ispreferred to the other wicks with respect to heat transfer ability andability against gravity of the earth.

Currently, a conventional method for making a sintered powder wickincludes filling a group of metal powders necessary to construct thewick into a hollow casing which has a closed end and an open end. Amandrel has been inserted into the casing through the open end of thecasing; the mandrel functions to hold the filled powders against aninner wall of the casing. Then, the casing with the powders is sinteredat high temperature for a specified time period to cause the powders todiffusion bond together to form the wick. In the method, it requires asintering temperature (or temperature range) suitable for the sinteringprocess.

The group of powders to be formed as the wick can be obtained bywell-known method such as mechanical grinding. Generally, the powdersthus obtained are a mixture of powders with different sizes due to atolerance in producing the powders. That is, the powders generally havea wide range of powder sizes. In addition, the proportion between thesedifferent sized powders is unknown before the sintering process. As aresult, it is difficult to determine the sintering temperature requiredby the sintering process and how long the sintering process should take.The general rule is that a lower sintering temperature and a shorterperiod of time are required to sinter and interconnect the small-sizedpowders in relative to the large-sized powders. If the sinteringtemperature is much too lower or the time to carry out the sinteringprocess is much shorter than it should be, the powders as applied toform the wick cannot be effectively diffusion bonded together. To thecontrary, if the sintering temperature is much too higher or the time ismuch longer than what is indeed required, the powders with relativelysmall sizes are apt to be overheated and melt. When the powders withrelatively small sizes have a large proportion in the group of powders,the wick accordingly formed will shrink significantly into a compact,high-density structure, noticeably reducing the pore size of the wickand causing the pores formed in the wick to be disconnected. Thedisconnected pores in the wick cannot provide a continuous passagewayfor the condensed liquid to return back along the wick.

Therefore, it is desirable to provide a method of manufacturing asintered powder wick by a sintering process. In the method, both therequired sintering temperature and the required time for the sinteringprocess can be easily determined and controlled.

SUMMARY OF INVENTION

The present invention relates to a method of manufacturing a wickstructure applicable in a heat pipe. A preferred method includes thefollowing steps: (1) providing a group of powders; (2) classifying thepowders into multiple sub-groups in terms of powder size, the sub-groupshaving powder sizes different from each other; (3) selecting at leastone sub-group of the powders; and (4) sintering the selected powders toform the wick structure.

In the method, if the selected powders are consisted of more than onesub-groups, these sub-groups are preferably mixed in a prescribedproportion by weight. Thus, the powders used to form the wick structureare confined to a relatively narrow range of powder sizes, and beforesintering, the proportion between these sub-groups is already known. Itis therefore easier to determine both the required sintering temperatureand the required time for the sintering process.

Other advantages and novel features of the present invention will becomemore apparent from the following detailed description of preferredembodiment when taken in conjunction with the accompanying drawings, inwhich:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart of a preferred method in accordance with thepresent invention, for manufacturing a wick structure applicable in aheat pipe;

FIG. 2 is a graph of the powder size distribution of a group of powdersas provided by the method of FIG. 1;

FIGS. 3-5 are schematic diagrams of one example of the presentinvention, showing in different stages of two sub-groups of the powdersin forming the wick structure by using the method of FIG. 1; and

FIGS. 6-7 are schematic diagrams of another example of the presentinvention, showing in different stages of one sub-group of the powdersin forming the wick structure by using the method of FIG. 1.

DETAILED DESCRIPTION

FIG. 1 shows a preferred method in accordance with the present inventionfor producing a porous wick structure that can be suitably applied toheat pipes or other heat transfer devices such as vapor chamber-basedheat spreaders. The wick structure is constructed from small-sizedpowders and a sintering process is required to form the wick structure.Firstly, a group of powders is provided, for example, by mechanicalgrinding a metal stock. Typically, the powders thus obtained areconsisted of powders with various different sizes. The sizes of thepowders may in fact range widely, for example, from several to hundredsof micrometers, based on the extent of preciseness and toleranceinvolved in the production of the powders. Thus, the powders as thusoriginally obtained are not suitable for being immediately used toconstruct the wick structure since it has a wide range of powder sizeand will raise uncertainty to the determination of the sinteringtemperature and time required by the sintering process. Thus, in thismethod, the powders are then classified into many sub-groups based ontheir powder sizes with each sub-group has a relatively narrow rangepowder sizes, as will be discussed in more details later. For easierunderstanding, in this embodiment, the powders used to construct thewick as originally obtained are presumed to have powder sizes rangingfrom 20 to 400 mesh. The “mesh” used herein represents the number ofopenings defined in per unit area of a standard screen. Standard screensare well known apparatus widely used to classify objects (such aspowders or the like) based on their sizes. If a standard screen is usedto classify powders, the number of openings in per unit area of thestandard screen is usually used to indicate the powder size of thepowders that pass through the standard screen. In this regard, it can beinferred that the powders having a powder size of 20 mesh are largerthan those having a powder size of 400 mesh. For illustrative purpose,the powder size distribution of the powders illustrated in FIG. 2 isgenerally in the form of a normal distribution.

Since the powders as originally obtained are generally not immediatelysuitable for the sintering process, the powders subsequently are dividedinto multiple sub-groups in terms of powder size. The aforementionedstandard screens will serve the purpose of the classification job.Specifically, the powders are brought to pass through a series ofstandard screens that have different meshes (i.e., openings) in per unitarea thereof. For example, if the powders are brought to sequentiallypass through a series of standard screens that have 200, 140, 100, and40 meshes respectively in per unit area thereof, the powders will beclassified five sub-groups, i.e., the sub-group A having a powder sizeof 200-220 mesh, the sub-group B having a powder size of 140-200 mesh,the sub-group C having a powder size of 100-140 mesh, the sub-group Dhaving a powder size of 40-100 mesh and the sub-group E having a powdersize of 20-40 mesh, as shown in FIG. 2. In this figure, the X-coordinaterepresents the powder size of the five sub-groups of the powders and theY-coordinate represents the powder distribution (by weight) of eachsub-group of the powders. It can be seen from this figure that amajority of the powders has a powder size of 100-140 mesh.

After the original group of powders is divided into these sub-groups, atleast one sub-group of the powders is selected to form the wickstructure. Selecting which group or groups of the powder to form thewick, however, is mainly based on what kind of characteristic the wickstructure is intended to have. For example, if the wick structure to beformed is intended to have a large capillary force, sub-groups A-C withsmall-sized powders are generally preferred. To the contrary, if thewick structure to be formed is intended to have a large permeability,then sub-groups C-E with large-sized powders are helpful. If multiplesub-groups of powders are selected, they should be mixed in certainproportions by weight, respectively. Finally, the selected sub-groups ofpowders are thoroughly mixed and sintered at a required temperature fora required period of time to form the intended wick structure.

An example of forming a wick structure in a heat pipe by selecting thepowders of sub-groups B and D is illustrated in FIGS. 3-5. The powders10 of sub-group D and the powders 20 of sub-group B are preferably mixedin a ratio by weight of 5:1 to 20:1. When the two sub-groups B and D aremixed, the small-sized powders 20 are nested in the spaces (not labeled)formed between the large-sized powders 10, as illustrated in FIG. 3.Although it is not shown in the drawings, it is well known by thoseskilled in the art that the powders 10, 20 after mixed are then filledinto a casing of the heat pipe and a mandrel is typically used to holdthe powders 10, 20 against an inner wall of the casing. The casing isthen placed into an oven and the powders 10, 20 are subsequentlysintered. Before the sintering process, the powder sizes of the selectedpowders (i.e., sub-groups B and D) and the proportion between them byweight are already known. Furthermore, the powders used to construct thewick structure are limited to the selected sub-groups each having arelatively narrower range of powder sizes. It is therefore easier todetermine the sintering temperature required by the sintering process.On this basis, the required time for the sintering process can also beeasily determined. As the sintering process is conducted under thedetermined sintering temperature and time, the small-sized powders 20 ofthe sub-group B become to melt and gradually turn into a molten state,as illustrated in FIG. 4. At this time, however, the large-sized powders10 of the sub-group D are almost intact except that their outer surfacesare melted. After the selected powders are sintered under the determinedsintering temperature for the determined period of time, the wickstructure is formed, wherein the large-sized powders 10 areinterconnected together by a plurality of necks 20′ which are formedfrom the small-sized powders 20, as illustrated in FIG. 5. Meanwhile, aplurality of voids 30 is formed between the large-sized powders 10.These voids 30 are communicated with each other so as to form acontinuous, liquid passageway. In this example, the small-sized powders20 are helpful to form the necks 20′ between the large-sized powders 10.In order for easier illustration and understanding, in the drawings thelarge-sized powders 10 are presumed unchanged throughout the sinteringprocess. In this example, since the proportion of the powders 20 of thesub-group B in the mixture is controlled, the melting of the powders 20will not cause the mixture to have an excessive shrinkage which mayresult in a disconnection between the voids 30. The required temperatureand time for the sintering process in this example are selected to meltthe powders 20 substantially entirely and the outer surfaces of thepowders 10.

Another example of forming a wick structure by selecting only one groupof the powders, such as group C, is illustrated in FIGS. 6-7. Thepowders 40 of group C have a powder size of 100-140 mesh, larger thanthe powder size of group B (140-200 mesh) but smaller than the powdersize of group D (40-100 mesh). In this case, since only one sub-group ofthe powders is selected to form the wick structure, the sinteringtemperature and time required in the sintering process is quite easy tobe determined. As the sintering process is conducted, the outer surfacesof the powders 40 become to melt, and meantime the powders 40 as a wholebecome to shrink and the contacting surface between neighboring powdersincreases, as illustrated in FIG. 4. After the sintering process, thepowders 40 are connected together by the melted outer surfaces thereof.Meanwhile, a plurality of voids 50 is defined between the powders 40.The required temperature and time for the sintering process in thisexample are selected to melt the outer surfaces of the powders 40. Theselected group C of the powders has a range of powder size which islocated at a middle of the distribution and includes a median of thedistribution of FIG. 2.

Following the above-mentioned examples, a wick structure may also beconstructed by selecting, for example, the sub-groups B and C, thesub-groups C and E, or the sub-groups B, C and D. A wick structurehaving a multi-layer structure may also be formed, for example, onelayer thereof being formed by selecting the powders of sub-groups B andD, and the other layer thereof being constructed from the group C, thusforming a gradient in capillary force between these layers of the wickstructure. Further, the powders used to construct the wick structure maybe copper powders, nickel powders, stainless steel powders, ceramicpowders or combinations thereof.

It is to be understood, however, that even though numerouscharacteristics and advantages of the present invention have been setforth in the foregoing description, together with details of thestructure and function of the invention, the disclosure is illustrativeonly, and changes may be made in detail, especially in matters of shape,size, and arrangement of parts within the principles of the invention tothe full extent indicated by the broad general meaning of the terms inwhich the appended claims are expressed.

1. A method of manufacturing a wick structure for a heat pipe comprisingsteps of: providing a group of powders; classifying the powders intomultiple sub-groups in terms of powder size, the sub-groups havingpowder sizes different from each other; selecting at least one sub-groupof the powders; and sintering the selected powders to form said wickstructure.
 2. The method of claim 1, wherein the powders as provided areone of copper powders, nickel powders, stainless steel powders andceramic powders.
 3. The method of claim 1, wherein the powders asprovided are classified into the multiple sub-groups by passing througha series of screens.
 4. The method of claim 1, wherein, when more thanone sub-groups of the powders are selected, the selected sub-groups ofthe powders are mixed in a prescribed ratio by weight.
 5. A method ofmanufacturing a wick structure for a heat pipe comprising steps of:providing multiple groups of powders with each group having an averagepowder size different from that of each of the other groups; selectingat least one group of powders from said multiple groups; and sinteringthe selected powders to form said wick structure.
 6. The method of claim5, further comprising a step of mixing the selected powders in aprescribed proportion by weight before sintering.
 7. The method of claim6, wherein the multiple groups of powders are obtained by classificationfrom an original group of powders having the multiple groups of powders.8. The method of claim 7, wherein the original group of powders isclassified into the multiple groups of powders by passing through aseries of screens.
 9. The method of claim 7, wherein the powders areceramic powders.
 10. The method of claim 7, wherein the powders are oneof copper powders, nickel powders and stainless steel powders.
 11. Amethod for forming a wick structure for a heat pipe, comprising:preparing a group of powders; separating the group of powders into aplurality of sub-groups of powders according to a size distribution ofthe powders, the sub-groups occupying different regions of the sizedistribution, respectively; selecting at least one of the sub-groups ofpowders and filling the selected powders into a casing of the heat pipe;and heating the casing and the selected powders to sinter the selectedpowders in the casing to thereby obtain the wick structure in the casingof the heat pipe.
 12. The method of claim 11, wherein two sub-groups ofpowders are selected, size ranges of the selected two sub-groups ofpowders are at discontinuous regions of the size distribution of thepowders.
 13. The method of claim 11, wherein the size distribution is anormal distribution, and one sub-group of powders is selected whichincludes a median of the normal distribution.
 14. The method of claim12, wherein a heating temperature and period of time for the heatingstep are selected to melt the selected powders of one of the twosub-groups substantially entirely, which have a smaller powder size thanthe selected powders of the other of the two sub-groups.
 15. The methodof claim 12, wherein one of the two sub-groups, which has a smallerpowder size has a weight less than that of the other of the twosub-groups, which has a larger powder size.
 16. The method of claim 15,wherein the other of the two sub-groups, which has the larger powdersize has a weight which is five to twenty times of that of the one ofthe two sub-groups, which has the smaller powder size.